1. Field of the Invention
The field of the present invention is light stripping. More particularly, the present invention relates to stripping of light propagating through the cladding of an optical fiber.
2. Background
Advances in laser technology have allowed for increasingly high powered systems. These systems include free space lasers, as well as lasers confined to waveguides, such as fiber lasers. Lasers in combination with fiber amplifiers have also allowed for very high peak power utilizing pulsed output. One advantage of high powered systems that utilize fiber is superior beam quality at the system output. Such systems can create beams having M2 values of closer to unity, thereby enabling high precision materials processing, among other applications.
Many high power fiber laser and fiber amplifier systems operate by coupling light from a pump source, such as another laser, into a fiber so as to excite an active core therein, and by spontaneous emission in the core or by directing a seed signal through the active core of the fiber so as to stimulate emission therealong. The seed signal accumulates power from the excited core and becomes amplified. Similar effects can occur with cladding light as power levels increase.
As more pump light is coupled into the fiber and the power output of the system increases, an increasing amount of unwanted residual light is observed in the cladding surrounding the core. Signal light, including highly amplified signal light, can leak out of the core and into the cladding as well. Moreover, as the system increases in power a third source of unwanted light occurs in the form of amplified spontaneous emission (ASE) increases as well. Consequently, as the power of these devices increases so do instabilities. As an approach to manage instabilities, pump strippers have been proposed to couple residual light out of the cladding. Various designs of pump strippers seek to divert higher numerical aperture cladding light out of the cladding using various means.
For example, the article “High power cladding light strippers” by Wetter et al., describes ways to attenuate cladding light. To minimize localized heating caused by stripping too fast with the intention of uniformly stripping light, high index polymers are used with gradually increasing refractive index. Also, different polymers having different indexes can be applied at different locations. In another example, U.S. Pat. No. 7,839,901 to Meleshkevich et al. discloses a cladding light stripper that includes two or more sub-regions of different materials each with a different refractive index. According to one embodiment described therein subsequent downstream regions have a refractive index that is higher than each previous region, and according to another embodiment described therein subsequent downstream regions have a refractive index that is lower than each previous region.
The aforementioned examples describe various ways to attenuate cladding light that unfortunately achieve mixed results and therefore fail to provide useful guidance for the light stripper designer. Thus, a need still remains for an innovation that will promote efforts to achieve a high power laser system utilizing fiber waveguides and cladding light strippers.